Industrial and Laboratory Pyrolyses

11 Kinetics of Product Formation in the H S-Promoted 2

Pyrolysis of 2-Methyl-2-pentene DAVID A. HUTCHINGS, K E N N E T H J. F R E C H , and FREDERIC H . HOPPSTOCK

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The Goodyear Tire & Rubber Co., Research Division, Akron, Ohio 44316

The d i s c o v e r y , by Z i e g l e r , e t . a l . , (1, 2) of s t e r e o s p e c i f i c p o l y m e r i z a t i o n , transformed isoprene into a monomer of great commercial importance. The continuing increase i n world demand for s y n t h e t i c poly-isoprene has catalyzed the search for l e s s c o s t l y isoprene processes. P r e s e n t l y , isoprene i s being produced v i a the dehydrogenation of a m y l e n e s , ( 3 , 4 , 5 ) the cracking of propylene dimer, (6,7,8) and by the i s o l a t i o n from r e f i n e r y C streams. (9,10,11). Isoprene production from isobutylene and formaldehyde appears promising, ( 1 2 , 1 3 , 1 4 ) with two p l a n t s operating s u c c e s s f u l l y i n the Soviet Union. The purpose of t h i s work is to define the mechanistic aspects of propylene dimer cracking in the presence of hydrogen s u l f i d e . 5

Experimental All p y r o l y s i s r e a c t i o n s were c a r r i e d out using the flow reactor described i n Figure 1. The r e a c t o r was constructed from s t a i n l e s s s t e e l and was adapted to i n l e t and e x i t tubing with commercially a v a i l a b l e s t a i n l e s s s t e e l compression fittings. L i q u i d hydrocarbon feed was introduced using glass syringes f i t t e d with t e f l o n plungers. The syringes were a c t i v a t e d using a commercially a v a i l a b l e gear pump. When steam was used as a d i l u e n t , its r a t e of i n t r o d u c t i o n was c o n t r o l l e d by i n t r o d u c i n g water in a manner s i m i l a r to that used f o r the hydrocarbon. When n i t r o g e n was used as a d i l u e n t , the input rate was measured using calibrated rotameters. The reactor was heated u s i n g staged r e s i s t a n c e heaters which r e c e i v e d t h e i r power v i a p r o p o r t i o n a l band c o n t r o l l e r s a c t i v a t e d by thermocouple sensors 178

In Industrial and Laboratory Pyrolyses; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

HUTCHINGS ET AL.

HgS-Promoted Pyrolysis of 2-Methyl-2-pentene

Hydrocarbon I n l e t

Diluent-H S Inlet Downloaded by PURDUE UNIVERSITY on June 3, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch011

2

Thermocouple Sensor E l e c t r i c a l Heater Reactor

Probe

Thermocouple

D i l u t i n g Nitrogen

\

Heated Sampline Line Figure 1. Reactor


INDUSTRIAL AND LABORATORY PYROLYSES


180

mounted on the s u r f a c e o f the t u b u l a r r e a c t o r . In o r der t o a t t a i n an i s o t h e r m a l p r o f i l e i n the r e a c t o r , a thermocouple probe t e c h n i q u e was used i n w h i c h the probe was moved a l o n g the c e n t r a l l o n g i t u d i n a l a x i s o f the r e a c t o r . By r e a d i n g t h e temperature a t f i x e d p o s i t i o n s i n s i d e t h e r e a c t o r , a temperature p r o f i l e a l o n g the l e n g t h o f the r e a c t o r was o b t a i n e d . By c o r r e l a t i n g i n t e r n a l temperature r e a d i n g s w i t h temperature c o n t r o l l e r s e t t i n g s , an i s o t h e r m a l p r o f i l e at any d e s i r e d temperature c o u l d be a t t a i n e d w i t h i n the r e a c tor. U s i n g t h i s t e c h n i q u e , a temperature d i f f e r e n c e a l o n g the r e a c t o r l e n g t h o f no g r e a t e r than ± 2°C c o u l d be r e a l i z e d w i t h m i n i m a l end e f f e c t s . D u r i n g t h e course o f a r e a c t i o n , p r o d u c t s f r o m the r e a c t o r p a s s e d i n t o the sample l o o p o f a h e a t e d gas sampling v a l v e . Prom the v a l v e , samples were i n t r o duced d i r e c t l y i n t o an a n a l y t i c a l gas chromatography In t h i s p a p e r , the v a r i a b l e s o f t e m p e r a t u r e , p r e s s u r e ( t h r o u g h the use o f d i l u e n t s ) , and f l o w r a t e have been u s e d to g a t h e r t h e k i n e t i c d a t a r e p o r t e d . Results

and D i s c u s s i o n

Thermal D e c o m p o s i t i o n o f 2 - M e t h y l - 2 - P e n t e n e ( 2 M P 2 ) • The f o l l o w i n g mechanism can be w r i t t e n f o r the f o r m a t i o n o f i s o p r e n e f r o m the p y r o l y s i s o f 2-methyl-2-pentene · Initiation : GH I

*1

G H « j G = G H G H 2 G H 3

GH ι •> CH«jC=CHGH2* + - C H - j

Propagation :

• C H 3 +

GHo R-(o6)

=

GH3C=GHCHGH GH

R*(l-oC)

=

3

•CH G=GHGH2CH3 2


11.

HUTCHiNGs

181

H S~Promoted Pyrolysis of 2-Methyl-2-pentene

ET AL.

2

Decomposition î CH3 =C •CH>|j C=CHCH CH3 2

GH =C-GH=GH 2

2

+

2

·0Η

3

Termination : •GH

+

3

Termination

R

CH^ +

· SH

Propagation : HS.

+ CH C=CHCH CH3

s—>

2

3

GHq

I R«

=

H S 2

+

R-

CHo

·

GH3G=CHGHGH3

I or

·0Η 0=0Η0Η2(2Η3 2


INDUSTRIAL AND LABORATORY

184

OH ο ι *

=

R.(cC)

PYROLYSES

.

CH C=GHGHCH 3

3

CEU ! 3 #

R*(l-c£)

= CH C=CHCH CH 2

2

3

Decomposition : GHo

GHo

•GH C=GHGH GH3 Downloaded by PURDUE UNIVERSITY on June 3, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch011

CH =C-GH=CH

2_>

2

2

2

2

+

·0Η

3

Termination : •SH

km

4- R»

—

> Termination products.

Based on the preceding mechanism, the f o l l o w i n g r a t e law may be d e r i v e d : Assumption Νο· 1 Rate o f I n i t i a t i o n = Rate of Termination (1)

kj

[2MP2]

=

k

[«SH] [ R . ]

T

Assumption No. 2 GHo

Rate o f

·CH G=GHCH CH 2

2

3

formation =

H

? 3 Rate o f

·σΗ 0=^^σΗ 0Η 2

2

3

decomposition. «

(2) (l-eÊîkp [ H S . ] [ 2 M P 2 ] S u b s t i t u t i n g Eq. 1 i n 2 : [•SH]

=

k

k

d I

=

k [R«] D

(1-. Three p o i n t s of i n t e r e s t may be noted. F i r s t , the l o g (H2S) versus l o g (k) p l o t s show a d e f i n i t e curvature at lower temperatures · At lower H2S concentrations, the r e a c t i o n order f o r H2S i s d e f i n i t e l y one h a l f , while at higher H2S l e v e l s the slope approaches one f o r the lower temperature. The o v e r a l l trend f o r the higher HgS

-

Figure 5.

log

k

H S-catalyzed isomerization of 2MP2 -» 4MP2 (order plots) x


190


2.6

Slope=.80 ^ 51+3°o

h-

Slope=.90

2.8

3.0

s.

Slope=1.0 516°G

> Slope=.6" X. 56U°C v

CO CNJ

>

3.2


bO Ο

1

*l 3.1+

Slor>e=.56 >v 587°C

3.6 3.8

ι

1

1.1+

1 1.6

1

1 log

Figure 6.

1

1.8

1

1

2.0

1 2.2

k

US-catalyzed isomerization of 2MP2 -» 4MP2 (order plots)

concentration data i s from an order of 1 . 0 to 0 . 5 as the r e a c t i o n temperature i s increased. A d u p l i c a t e set of experiments, Figure 6, confirms the order trend with temperature. Based on the work of Maecoll and Ross, (16) the f o l l o w i n g type o f r e a c t i o n sequence can be p o s t u l a t e d f o r l\M?2 formation from 2MP2: H 0=0' CH-:

CH3-C H*

... s

CH

3;o) H Ή-^£ \

H

I f we consider the above i s o m e r i z a t i o n process to have a low a c t i v a t i o n energy and A f a c t o r , w h i l e the r a d i c a l chain process proposed e a r l i e r proceeds v i a a higher a c t i v a t i o n energy process, with a h i g h e r A f a c t o r , then the molecular ( f i r s t order) process should be observed at lower temperatures w i t h a gradual trans i t i o n being observed to the r a d i c a l chain process as the temperature i s i n c r e a s e d , p r o v i d i n g the r a t e s f o r



11.

HUTCHiNGS E T A L .

H S-Promoted Pyrolysis of 2-Methyl-2-pentene X

191

these processes are comparable. This i s the trend which i s observed i n the data presented i n Figure In a d d i t i o n , at lower H2S l e v e l s , the r a d i c a l chain process would be favored over the molecular process ( h a l f order dependency of H^S f o r the r a d i c a l process as opposed t o f i r s t order f o r the molecular p r o c e s s ) . The trend of order from one-half to f i r s t - o r d e r shown i n Figure 5 supports t h i s hypothesis. The Arrhenius p l o t presented i n Figure 7 was derived from a s e r i e s of rates (see dotted l i n e i n Figure 6) f o r 2MP2 > l\M?2 i s o m e r i z a t i o n at d i f f e r e n t temperatures, but at a f i x e d l e v e l . I t w i l l be noted that the E c a l c u l a t e d from the Arrhenius p l o t i s 3 6 . 8 kcal/mole, t h i s being a value between the t h e o r e t i c a l E s f o r the molecular and r a d i c a l chain i s o m e r i z a t i o n processes. a

f

a

Methyl Pentadienes Formation* Rate data f o r methyl pentadienes formation i n the presence of H2S are presented i n Figure 8 . At both temperatures, methyl pentadienes formation i s found to b e 0 . 3 order i n H2S. The observed E f o r the formation o f methyl pentadienes based on these data i s 2 6 . 5 k c a l / mole. When one considers the p o s s i b l e free r a d i c a l homogeneous mechanism f o r methyl pentadienes formation, the f o l l o w i n g mechanism may be proposed: a

1.ο

ι.2 1

M* Ο Μ

1.6

1.8

2.0

1.16

1.20

( D a t a Taken a t [E^S] Figure 7.

1.21+ 1/Τ χ 1 θ 3

1.28

= 1.6 χ 10~3 m o l e s / l i t e r )

Arrhenius plot of HgS-catalyzed 2MP2 isomeri zation to 4MP2 data



192


2.8

Figure 8. Methyl pentadiene rate data (order in H,S)

Methyl Pentad!enes

3.0

3·2 ~

3.1;

3·6 η K L 2 J

L

0

H

S

Formation i n the Presence o f H?S

Initiation: CH

ÇH

3

CH C=CHCH CH 3

2

3

£ — > CH C=CHGH ' +

3

3

2

'CH

3

(2MP2) Transfer: HS 2

+

Propagation:

T l > a n 3

.CH

3

Q

Η3·

>

E

+

CH^ G E

HS. + CH C=GHCH CH 3

2

^

H S + CH C=CHCHCH

3

2

3

3

Decomposition: CH C=CHCHCH 3

3

3·8

r

> CH =C-CH=CHCH + H* 2

3


11.

HUTCHiNGS

HS-Promoted Pyrolysis of 2-Methyl-2-pentene

ET AL.

t

193

Termination: le • SH

+

T

R.

k![2MP2]

=

-—> Termination products.

k [.SH][R.] T

CH3 k [HS'J[2MP2]

=

p

k [CH3C=CHCHCH ] + d

3

CH

3

k . [ H S ] [cH C=CHCHCH ] Downloaded by PURDUE UNIVERSITY on June 3, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch011

p

2

3

3

kppîS»] [ 2 M P 2 ] kp [ H S ] + k. .

[CH3G=CHÔHGH3]=

2

T

D

ÇH3 Assuming R. « CH3C=CHCHCH3, k

[HS.]

I

1

/

Γ

2

1/2

=

k

k

1/2

[k

p

[H S]

k

S

+

k

Rate o f Methylpentadiene

d) Formation = k ^ ( R O 1/2

kjkp

[2MP2]

d k

T

( k

+ k j

[2MP2]

T< p[ 2 ] H

2

1/2

I p

k

Rate = k

11/2

p[ 2 ] H

S

+

k

d>

Where HoS = 0

Rate =

k^jkp

1/2 [2MP2]

Based on t h e above mechanism, the r e a c t i o n l e a d i n g t o me t h y 1 p e n t a d i e n e s f o r m a t i o n s h o u l d be f i r s t o r d e r i n 2MP2, have a h i g h a c t i v a t i o n e n e r g y , i . e . , 60 k c a l / m o l e , a n d show a n e g a t i v e 1/2 o r d e r d e p e n d e n c y o n H£S. Our o b s e r v e d d a t a i n d i c a t e t h a t a p o r t i o n o f t h e methyl p e n t a d i e n e s f o r m i n g r e a c t i o n may be h e t e r o g e n e o u s i n origin. I f one c o n s i d e r s the f o l l o w i n g l o w - e n e r g y process,



194

CH I · CH C=CHCHCH3 + H S c=CHcr"~ ""

kg -> H + —

3

3

•SH

2

2

the r a t e equation becomes: Rate of methyl pentadiene formation =


kg

[R.][H S] 2

k

"

p[ 2S] > k H

d

, 1/2.

Rate = kc

k k T

E

a

[H S] 2

p

1 / 2

[>V\]

= ^ 3 0 kcal/mole

The Role o f E?S i n Modifying the 2 M P 2 Cracking Product Distributions» Product weight percentages f o r H S-promoted and thermal 2 M P 2 c r a c k i n g rims at compa r a b l e conversion l e v e l s are presented i n Table I . I t w i l l be observed that f o r the HoS-promoted runs, the products d e r i v e d from HpS i n t e r a c t i o n s w i t h 2 M P 2 (2MP1, methyl pentadienes and IJMP2) are major products. I n the thermal case, products derived from r a d i c a l a d d i t i o n r e a c t i o n s predominate. These r e a c t i o n s are as follows : 2

Methyl Butene Formation: I

I

• C H 3 + CH3C=CHCH CH3

3

> CH3C-CHCH2CH3

2

CH3

i

J

I

CH3G-GHGH2CH3 CH

>

σΗ σ=σΗσΗ3 3

+

GH3CH * 2

3

Isobutylene Formation: CH3

Η· + 01130=0HOH GH3 2

> 0H30-CH0H20H3 2


11.

HUTCHiNGS E T A L .

H S-Promoted Pyrolysis of 2-Methyl-2-pentene 2

GH I 0H30-0H 0H2CH3

OH ο I > 0H 0=CH

195

J

2

3

2

+ 0Η 0Η · 3

2

The m e t h y l r a d i c a l s a n d h y d r o g e n a t o m s p r e s e n t i n t h e thermal system can also i n t e r a c t w i t h product isoprene to give r i s e to a d d i t i o n a l p r o d u c t s . Thus, the r o l e o f H S a p p e a r s t o be t h a t o f a h y d r o g e n a n d m e t h y l r a d i c a l scavenger. The r e s u l t i n g *SE r a d i c a l s , w h i l e they can propagate the r e a c t i o n , cannot p a r t i c i p a t e i n the a f o r e m e n t i o n e d r a d i c a l a d d i t i o n r e a c t i o n s w h i c h can l e a d to unwanted b y - p r o d u c t s .


2

Sequential Products. T h e p r o d u c t s whose f o r m a t i o n a r e e n h a n c e d b y H S (2MP1, 1|MP2, m e t h y l p e n t a dienes) can r e a c t f u r t h e r to produce isoprene a t varying efficiencies. These m a t e r i a l s , h o w e v e r , a l s o give other by-products : 2

Compound 2MP1 ILMP2 Methyl pentadienes

Main Reaction i s o b u t y l e n e + e t h a n e o r ethylene piperylene + cyclopentadiene methyl cyclopentadiene

Conclusions When 2 - m e t h y l - 2 - p e n t e n e i s p y r o l y z e d i n t h e presence o f HpS, the mechanism o f i s o p r e n e f o r m a t i o n i s n o t changed. Hydrogen s u l f i d e serves to m o d i f y the b y - p r o d u c t mechanism p a t h w a y s . The m a j o r b e n e f i c i a l r o l e o f H S i s as a h y d r o g e n atom s c a v e n g e r , w h i c h decreases the undesirable side reactions r e s u l t i n g f r o m h y d r o g e n atom a d d i t i o n . A s i d e f r o m t h e s e b e n e f i t s , H2S w o r k s t o t h e d e t r i m e n t o f t h e i s o p r e n e f o r m a t i o n i n s e v e r a l r e a c t i o n s b y p r o m o t i n g a number o f homogeneous d o u b l e b o n d i s o m e r i z a t i o n r e a c t i o n s i n v o l v i n g 2-methyl-2-pentene producing o l e f i n s which are poorer isoprene precursors. 2

Acknowledgements The a u t h o r s w i s h t o t h a n k D r . S . W . B e n s o n a n d D r . J . E . T a y l o r f o r t h e i r h e l p f u l d i s c u s s i o n s o n the material presented i n this paper.


196

INDUSTRIAL A N D LABORATORY

PYROLYSES

Literature Cited 1. Z i e g l e r , Κ., Holzcamp, Ε., Breil, H., and M a r t i n , H., Angew. C h e m . , 67, 541 (1955); Ziegler, Κ., Belgium P a t e n t 533,362 (November 16, 1954). 2. G o o d r i c h - G u l f C h e m i c a l Co., B e l g i u m P a t e n t 543,292, December 2, 1955. 3. Dow C h e m i c a l , British P a t e n t 746,611, March 14,

1956. 4.

P i t z e r , E. W., U . S . P a t e n t 2,866,790, December

30,


1958. 5.

P i t z e r , E. W., U.S. P a t e n t 2,866,791, December

30,

1958. 6.

Goodyear T i r e & Rubber

Co.,

British

Patent,

July

13, 1960. 7.

10.

Goodyear T i r e & Rubber Co., British P a t e n t 832,475, August 13, 1960. Goodyear T i r e & Rubber Co., British P a t e n t 868,566, May 17, 1961. K e l l e y , R. T. et al. U . S . P a t e n t 3,012,947, Dec ember 12, 1961. Hachmuth, Κ. Η., U.S. P a t e n t 3,038,016, June 5,

11.

K i n g , R. W. et al, U.S. P a t e n t 3,301,915, J a n u a r y

8. 9.

1962. 31, 1967. 12.

Hellin, M. et al, British P a t e n t 884,804, Decem ber 20, 1961. 13. M i t s u t a n i , Α., U.S. P a t e n t 3,284,533, November 8,

1966. 14. 15. 16.

M i t s u t a n i , Α., et al, U.S. P a t e n t 3,284,534, November 8, 1966. Benson, S . W., Thermochemical Kinetics, John Wiley & Sons, New Y o r k , 1968. M a e c o l l , A . and R o s s , R. Α., J. Am. Chem. Soc.,

87, 4997 (1965).


Industrial and Laboratory Pyrolyses

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